Very generally, a taximeter is an apparatus whose object is to indicate the price to be paid for a trip made by the taxi, this price depending on several parameters, including, inter alia, the distance covered by the taxi, i.e. ultimately, the number of wheel turns made by this vehicle during the trip.
The sensor used for measuring this number of wheel turns is in that case the sensor normally associated with the dashboard of the vehicle and therefore connected to the speedometer which indicates both the instantaneous speed of this vehicle and the mileage covered thereby.
Virtually all modern vehicles are equipped with an electromagnetic or electronic sensor for sensing the number of wheel turns, called "electronic sensor", which is provided at the level of the gear box and which is equipped with an output connector on which is connected a cable which collects and conveys the electrical pulses representative of the number of wheel turns to the speedometer which equips the dashboard. The dashboard is in that case conventionally equipped with an auxiliary output, which is electrically connected in parallel on this cable, and on which is connected the corresponding input of the taximeter: the pulses which are conveyed on this cable therefore supply the speedometer of the vehicle and the taximeter simultaneously.
The situation is relatively similar concerning the tachographs with which trucks or lorries must obligatorily be fitted and which, as is known, serve at least to register on a disc the speed of the truck, the miles covered and the driver's work time. In that case, it is generally provided to interpose on the cable which connects the sensor to the speedometer, an electronic adapter which is supplied by the battery through a fuse and which delivers pulses, deducted from those delivered by the sensor, in the direction of the tachograph to which this adapter is connected by an electric cable provided to that end.
These two types of metering apparatus, taximeters or tachographs, are sealed with lead by the Weights and Measures Department, but, unfortunately, this is not sufficient to avoid fraud which is becoming increasingly frequent.
One form of fraud which is frequently encountered at the present time consists in connecting, between the sensor and the taximeter or tachograph, a small auxiliary pulse generator whose frequency is controlled by that of the pulses of the sensor and which consequently delivers pulses whose frequency differs, in a defined ratio which is for example of the order of 1.2, from that of the pulses delivered by this sensor.
In the case of a taximeter for example, the frequency of the pulses which are effectively applied thereto is in that case chosen to be 1.2 times greater than that of the pulses delivered by the sensor, with the result that everything happens as if the taxi is advancing at a speed 1.2 times greater than its real speed, this passing onto the displayed price which is then 1.2 times greater than the price that the client ought in fact to pay.
On the contrary, in the case of a tachograph, the defrauder's apparatus is then adjusted to deliver pulses of frequency 1.2 times less than that of the pulses of the sensor and, for the tachograph, everything happens as if the truck were advancing at a speed 1.2 times less than its real speed.
This ratio can, of course, be manually adjusted most of the time.
It is an object of the invention to prevent any fraud based on a falsification of the speed data which is delivered to the taximeter or to the tachograph.